<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
      <img id="Img" src="Img" alt="验证码"  />  
      <a href="#" onclick="reImg();">看不清，换一张</a>  
</body>
<script type="text/javascript">
            function reImg(){  
                var img = document.getElementById("Img");  
                img.src = "Img?rnd=" + Math.random();  
            }  
</script>
</html>